Our current and continuing investigation in general terms is directed toward an understanding of human purine metabolism and its regulation by the use of normal and mutant human skin fibroblasts in tissue culture. We have previously been involved in elucidating certain genetically determined alterations of purine metabolism which lead to excessive uric acid synthesis and hyperuricemia in man. These include hypoxanthine- guanine phosphoribosyltransferase deficiency, adenine phosphoribosyltransferase deficiency, and a genetic alteration leading to a PRPP amidotransferase with a defective regulator site. We are now proceeding along four main lines of inquiry. 1) A detailed study of the physical biochemistry of hypoxanthine-guanine phosphoribosyltransferase in cultured fibroblasts, 2) A detailed study of the regulatory properties of the glutamine phosphoribosylpyrophosphate amidotransferase present in normal fibroblasts as well as certain human mutants. 3) A detailed study of the mechanisms by which each of these known enzymatic defects alters the rate of purine biosynthesis de novo in the mutant cells, and 4) A continued search for other defective enzymes within the purine pathway in fibroblasts derived from hyperuricemic patients who have none of the alterations previously described.